During integrated circuit fabrication, a plurality of integrated circuits (dies) are formed on a single semiconductor wafer simultaneously by a series of material deposition and removal processes. The individual dies are then separated from the wafer, in a process called dicing. Typical wafer dicing involves attaching the wafer to an adhesive dicing tape, followed by separation of the dies from each other and the wafer. Separation is typically achieved by dicing with a circular saw or by scribing and breaking the wafer (if the wafer is crystalline). If the dies contain micro electromechanical systems (MEMS), the latter method is typically used, because use of a saw would produce too many particles that could interfere with the device. Even using the scribe-and-break technique, the die ejection process from the tape is non-trivial.
FIG. 1 shows a die 100 including a MEMS structure 104. The exemplary MEMS 104 is a mirror attached to the die 100 which pivots about an axis 106 under control of the circuitry 102, and is also connected to the die by a plurality small springs, not shown. The springs (not shown) bias the mirror to the horizontal position of FIG. 2. The MEMS 104 may be, for example, a mirror sized between about 200 microns and 300 microns, connected to the outer portion of the die 100 by 2 springs (not shown). Although an example of a mirror is described, this discussion applies equally to other types of MEMS with at least one moving part.
A common die-ejection procedure includes placing the adhesive tape holding the dies on top of a base that includes an array of sharp-pointed pyramid-shaped structures. A vacuum is applied between the tape and the base. The tape is thus pulled down, minimizing the contact area with the die and facilitating its removal. An example of such a machine is a Model 4800 Die Ejection Grid machine manufactured by Semiconductor Equipment Corp. of Moorpark, Calif.
FIG. 2 shows a cross section of the die 100 of FIG. 1. The die 100 is attached to dicing tape 120, which is placed on the base 110 having pyramid-shaped structures.
During the ejection procedure, the tape 120 charges up due to charge separation at the interface and the electrostatic force on the MEMS device 104 can attract the moving part of the MEMS, resulting in the moving part becoming irreversibly stuck to the adhesive tape, as shown in FIG. 3. When the die 100 is subsequently removed from the tape 120, the MEMS 104 remains on the tape, rendering the die useless.
Although ultraviolet (UV) curable dicing tape has been developed to release items attached thereto upon curing the tape, even UV curable tapes retain a small amount of adhesion after exposure to UV radiation. This is enough to tear the MEMS 104 away from the outer portion 100 of the die.
Another attempt at protecting the MEMS involved the use of “anti-static” tapes. However, because of the small spring constant of the springs (not shown), the anti-static tapes may still develop enough charge to draw the MEMS to the tape.
In addition to the static electric charge, the sudden displacement of the tape 120 upon application of the vacuum can create an air flow (referred to herein as the “pneumatic effect”) that tends to move the movable part 104 of the MEMS into contact with the tape 120.
Improved techniques are desired.